The subject invention is generally directed to copying holograms, and more particularly to a technique for simultaneous copying of two holograms into a single hologram recording layer.
In a number of applications such as laser eye protection and head-up displays, it may be desirable to utilize a hologram structure that has a bandwidth that is greater than that typically available from a single hologram. One approach to achieving a wider bandwidth hologram is to utilize a hologram structure having two holograms, and it would generally appear that the two holograms can be in the same hologram recording layer, or they can be in different layers that are individually exposed and later laminated.
However, recording two holograms in a single recording layer presents various problems. If the holograms are exposed simultaneously, interaction between respective sets of exposure beams (i.e., interaction of any beam from one set with any beam from another set) will produce undesirable crosstalk holograms that would reconstruct at unacceptably high intensities. Recording the two overlapping holograms in a single recording layer pursuant to sequential exposures can be performed with dichromated gelatin (DCG) and silver halide recording materials without significant degradation, but not with a photopolymer. A consideration with successive exposures, however, is the requirement for more handling, and for DCG a change of shrinkage factor for the subsequent exposures. While non-overlapping adjacent holograms can be successively recorded in a photopolymer, surface distortion including a ridge and/or a trough occurs around each exposure area, which might not be amenable to removal by cutting.
Recording individual holograms on respective recording layers which are later laminated together involves the extra steps and costs associated with lamination and the additional film. This approach would work for a master hologram or a small number of copies, but is inconvenient as well as uneconomical for a large number of copies.
Another approach to the need for wider bandwidth is to artificially broaden the hologram in processing to obtain a wider bandwidth or even two distinct peaks. This requires critical processing for any type of recording material.